1. Field of the Invention
The present invention relates to an electrodialysis apparatus and method used for transferring dissolved salts or impurities from a waste or other solution (commonly known as a diluate solution) into a concentrate solution, and more particularly, to a new and improved electrodialysis apparatus and electrodialysis process that enables the transfer of such dissolved salts or impurities from such a waste or other diluate solution into a concentrate solution without the precipitation of certain insoluble compounds adjacent the cathode of the electrodialysis apparatus or the liberation of chlorine gas at the anode of the electrodialysis apparatus or the liberation of ammonia at the cathode of the electrodialysis apparatus.
2. Background of the Invention
Electrodialysis (ED) is used in connection with the separation of dissolved salts or other impurities from one aqueous solution to another aqueous solution. The separation of these dissolved salts or other impurities results from ion migration through semi-permeable, ion-selective membranes under the influence of an applied direct current field that is established between a cathode (negative potential electrode) and an anode (positive potential electrode). The membranes may be selective for monovalent or multivalent ions depending on whether separation is desired between monovalent or multivalent cations and/or anions. The separation process results in a salt or impurity concentrated stream (known as a concentrate or brine) and in a salt or impurity depleted stream (known as a diluate). The concentrate and diluate streams flow in solution compartments in the electrodialysis apparatus that are disposed between the anode and cathode and that are separated by alternating cation and anion selective membranes. The outer most compartments adjacent the anode and cathode electrodes have a recirculating electrode-rinse solution flowing therethrough to maintain the cathode and anode electrodes clean. A schematic of one type of electrodialysis apparatus is illustrated in FIG. 1.
The electrodialysis apparatus 20 shown in FIG. 1 has a series of alternating cation semi-permeable, ion-selective membranes C and anion semipermeable, ion-selective membranes A disposed between a positive DC potential anode electrode 22 and a negative DC potential cathode electrode 24. The cation-selective membranes C and the anion selective membranes A form compartments therebetween. As indicated in FIG. 1, concentrate and diluate solutions flow as indicated respectively by arrows 26 and 28 through adjacent compartments such that the concentrate and diluate solutions are separated from each other by the ion-selective membranes. The diluate solutions may contain salts (such as sodium chloride (NaCl)) or impurities (such as sodium chloride (NaCl) in acidic magnesium chloride (MgCl2) solutions or calcium chloride (CaCl2) and magnesium chloride (MgCl2) in sodium chloride (NaCl) and potassium chloride (KCl) solutions). Due to the potential maintained across each of the compartments and cation and anion selective membranes separating the compartments, cations (such as acid (H), sodium (Na), magnesium (Mg), calcium (Ca) and potassium (K)) and anions (such as chloride (Cl)) as well as water (hydration shell and osmosis) will tend to migrate from the diluate solution to the concentrate solution. Once these cations and anions are in the concentrate solution, they can be recovered and used for commercial purposes. Additionally, the purified or salt-depleted diluate solution also may have an increased commercial value.
As further indicated in the schematic of FIG. 1, an electrode rinse solution is circulated in an outer most compartment 30 adjacent to the anode 22 and an outer most compartment 32 adjacent to the cathode 24. During the electrodialysis process, hydrogen tends to be evolved at the cathode 24 and oxygen tends to be evolved at the anode 22. As a result, the pH level in the electrode rinse solution that is circulating through the compartment 32 adjacent the cathode 24 increases while the pH level of the electrode rinse solution that is circulating through the compartment 30 adjacent the anode 22 decreases. In view of the fact that the electrode rinse solution is mixed after flowing through the compartments 30 and 32, the increase and decrease in the pH level of the electrode rinse solution used in the electrodialysis apparatus 20 tends to be neutralized.
The circulation of fluids through an electrodialysis apparatus, such as the electrodialysis apparatus 20, is shown schematically in FIG. 2 of the drawings. As shown therein, the diluate solution is pumped from and to a diluate tank 34 through the diluate compartments formed between the cation C and anion A selective membranes (labeled ED stack in FIG. 2) by a diluate pump 36. In a like manner, the concentrate solution is pumped from and to a concentrate tank 38 through the concentrate compartments formed between the cation C and anion A selective membranes (ED stack) by a concentrate pump 40. As the diluate solution flows through the diluate compartments, cations, anions and water from the diluate solution migrate through the cation C and anion A selective membranes to the concentrate solution. In addition, an electrode rinse solution is pumped by an electrode rinse solution pump 42 from an electrode rinse solution tank 44 through the compartments 30 and 32 adjacent respectively of the anode 22 and the cathode 24. As previously indicated, any changes in the pH level of this electrode rinse solution is neutralized when the electrode rinse solution is mixed together as it flows from the compartments 30 and 32 to the electrode rinse solution tank 44.
While the use of one electrode rinse solution has the advantage that changes to the pH level of the solution as it flows through the compartments 30 and 32 are neutralized, precipitates tend to form on the ion-selective membrane forming a side of the compartment 32 adjacent to the cathode 24. In this regard, reference can be made to FIGS. 3-8 of the drawings. These figures diagrammatically illustrate the types of precipitates that might be formed on a cation selective membrane that is disposed on a side of the compartment 32 adjacent the cathode 24. It should be noted that the membrane arrangement in FIGS. 3-8 is different from the arrangement in FIG. 1, but this has no bearing on the present invention.
FIG. 3 illustrates what might occur when an electrode rinse solution of sodium hydroxide (NaOH) is used in an electrodialysis apparatus in which is treated pickle liquors from the steel industries which liquors contain chloride, iron and/or manganese ions. The NaOH electrode rinse solution is generally a basic solution having a pH level of about 14. While the electrodialysis apparatus enables the removal of iron and/or manganese from those liquors, iron hydroxide (Fe(OH)3) and/or manganese hydroxide (Mn(OH)3) precipitates tend to form on the diluate side of the cation ion selective membrane on the side of the compartment 32. The reason that such precipitates form is due to the migration of iron and/or manganese ions towards the cathode 24 that react with the hydroxide ions of the NaOH electrode rinse solution flowing through the compartment 32. Such precipitates on the cation ion selective membrane are detrimental to the functioning of the electrodialysis apparatus 20 because the precipitates tend to block or interfere with the transfer of iron or manganese ions across that membrane.
FIG. 4 similarly illustrates what might occur when the NaOH electrode rinse solution is used in an electrodialysis apparatus in which is being treated effluents from the pulp and paper industry. Those effluents contain chloride and calcium ions. When these effluents are treated in an electrodialysis apparatus (such as the electrodialysis apparatus 20) to remove chloride and calcium ions, calcium hydroxide (Ca(OH)2) precipitates tend to form on the diluate side of the cation ion selective membrane on the side of the compartment 32. The reason that such precipitates form is due to the migration of calcium ions towards the cathode 24 that react with the hydroxide ions of the NaOH electrode rinse solution flowing through the compartment 32. Such calcium hydroxide precipitates on the cation ion selective membrane are detrimental to the functioning of the electrodialysis apparatus 20 because the precipitates tend to block or interfere with the transfer of the calcium ions across that membrane.
The detrimental formation of such precipitates occurs even if a more neutral, but basic type electrode rinse solution (pH level greater than 7) is used. In this regard, reference can be made to FIGS. 5 and 6 of the drawings. As illustrated in those figures, sodium nitrate can be used as the electrode rinse solution (in the alternative sodium sulfate likewise can be used). Such an electrode rinse solution has a pH level that is greater than 7. However, the pH level of the electrode rinse solution and the diluate tend to increase during the electrodialysis processing of pickle liquors causing the formation of iron and manganese hydroxide precipitates to be formed (FIG. 5) or during the electrodialysis processing of effluents from the pulp and paper industry causing the formation of calcium hydroxide precipitates to be formed (FIG. 6). In both of these instances, the precipitates form on the side of the membrane forming the side of the compartment 32.
The pH level of the NaNO3 electrode rinse solution can be lowered to the acidic range (a pH level below 7) using sulfuric acid. This tends to eliminate the problem of the formation of precipitates of iron and manganese hydroxide in the case of the electrodialysis processing of pickle liquors or the formation of precipitates of calcium hydroxide in the case of the electrodialysis processing of effluents from the pulp and paper industry. However, the use of such an acidic electrode rinse solution tends to cause another problem, i.e., chlorine gas liberation at the anode. In this regard, reference can be made to FIGS. 7 and 8 of the drawings. As illustrated in those FIGS. 7 and 8, the use of an acidic electrode rinse solution flowing through the compartment 30 adjacent the anode 22 tends to cause the migration of chloride ions from the diluate solution through the cation selective membrane forming the side of the compartment 30 to the electrode rinse solution. This happens because the H+ ions in the acidic electrode rinse solution react with the negative functional groups on the cation selective membrane on the side of the compartment 30 rendering the membrane useless as a cation exchange membrane. In particular, the functional groups on the cation exchange membrane become neutral after reacting with the H+ ions and as a result, the membrane is no longer ion selective such that the Clxe2x88x92 ions are allowed to pass through.
Similar type problems discussed above with respect to the electrodialysis processing of effluents from the pulp and paper industry and of pickle liquors also occur when the electrodialysis apparatus 20 is used to process magnesium chloride (MgCl2) rich aqueous streams. Such streams may be produced in two ways: (1) Elemental magnesium (Mg) can be remelted under a salt blanket. When is so remelted, some magnesium (Mg) is left in the salt and when the salt is solubilized, an aqueous stream containing magnesium (Mg), sodium (Na) and calcium (Ca) is produced. (2) Impure MgO.SiO2 or other source of magnesium (Mg) is leached in a HCl solution resulting in the production of an aqueous stream containing Mg, H (acid), Na and Ca, in which Ca, H (acid), and Na are considered impurities. When these streams are processed by electrodialysis to remove H (acid) and Na utilizing monovalent selective membranes to thereby produce high quality magnesium chloride aqueous solution, and the same electrode rinse solution is used for the anode and cathode electrodes, either chlorine gas is liberated from the electrode rinse solution (such as an acidified NaNO3 electrode rinse solution) flowing in the compartment 30 adjacent the anode 22 or a magnesium hydroxide precipitate forms on the ion selective membrane adjacent the cathode 24 when the electrode rinse solution is a more basic solution (for example, alkaline NaNO3 or NaOH solutions).
Accordingly, it is an object of the present invention to provide a new and improved electrodialysis apparatus and method that eliminates the formation of detrimental precipitates on ion selective membranes adjacent the cathode of the electrodialysis apparatus and/or the liberation of chlorine gas adjacent the anode of the electrodialysis apparatus.
It is another object of the present invention to provide a new and improved electrodialysis apparatus and a method that enables the electrodialysis processing of an aqueous diluate and concentrate solutions so that impurities in the diluate solution migrate to the concentrate, or vice versa, without the formation of detrimental precipitates on ion selective membranes adjacent the cathode of the electrodialysis apparatus and/or the liberation of chlorine gas at the anode of the electrodialysis apparatus.
It is still another object of the present invention to provide a new and improved electrodialysis apparatus and method wherein separate anode and cathode electrode rinse solutions are utilized so that detrimental precipitates are not formed on ion selective membranes adjacent the cathode of the electrodialysis apparatus and/or chlorine gas is not liberated at the anode of the electrodialysis apparatus.
In accordance with these and many other objects of the present invention, an electrodialysis apparatus includes a series or stack of alternating cation semi-permeable, ion-selective membranes and anion semi-permeable, ion-selective membranes disposed between a positive DC potential anode electrode and a negative DC potential cathode electrode. The ion-selective membranes may or may not be monovalent selective. The cation selective membranes and the anion selective membranes form compartments therebetween through alternate compartments of which a concentrate solution is circulated and through other compartments of which a diluate solution is circulated such that the concentrate and diluate solutions are separated from each other by the ion selective membranes. The diluate solutions may contain salts (for example NaCl) or impurities (for example, sodium chloride (NaCl) and acid (HCl) in magnesium chloride (MgCl2), or calcium chloride (CaCl2) in sodium chloride (NaCl) and potassium chloride (KCl) solutions). Due to the potential maintained across each of the compartments and the cation and anion selective membranes separating the compartments, cations (such as acid (H), sodium (Na), magnesium (Mg), calcium (Ca) and potassium (K)) and anions (such as chloride (Cl)) as well as water (hydration shell and osmosis) will migrate from the diluate solution to the concentrate solution. Once these cations and anions are in the concentrate solution, they can be recovered and used for commercial purposes. Additionally, the purified or salt-depleted diluate solution may have an increased commercial value.
An anode electrode rinse solution is circulated through an outer most compartment adjacent to the anode electrode and a separate cathode electrode rinse solution is circulated through an outer most compartment adjacent to the cathode electrode. In one embodiment of the present invention, the anode electrode rinse solution is sodium hydroxide (NaOH) maintained at a basic pH level that is near 14 and the cathode electrode rinse solution is sodium chloride (NaCl) and/or potassium chloride (KCl) that are maintained at an acidic pH level of about 6. The use of these separate electrode rinse solutions tend to eliminate problems that occur when the same electrode rinse solution is used for both the anode and cathode electrodes. In particular, the acidic cathode electrode rinse solution tends to prevent the formation of detrimental precipitates (such as iron, manganese and magnesium hydroxides) on the membrane adjacent the cathode electrode and the basic anode electrode rinse solution tends to prevent the liberation of chlorine gas from adjacent the anode electrode or the liberation of ammonia from adjacent the cathode electrode depending on the impurities in the diluate solution. During the electrodialysis process, hydrogen tends to be evolved at the cathode and oxygen tends to be evolved at the anode. As a result, the pH level in the cathode electrode rinse solution that is circulating through the compartment adjacent the cathode tends to be increased so acid has to be continuously added to the cathode electrode rinse solution to maintain its acidic pH level and the pH level of the anode electrode rinse solution that is circulating through the compartment adjacent the anode tends to be decreased so a base has to be continuously added to the anode electrode rinse solution to maintain its basic pH level.